Freeze drying system



ug. 13, 1968 E. G. SCHEIBf-:L 3,396,475

FREEZE DRYING SYSTEM Filed Jan. 10, 1966 & fief/v ,/9

United States Patent O 3,396,475 FREEZE DRYING SYSTEM Edward GeorgeScheibel, 75 Harrison Ave., Montclair, NJ. 07042 Filed Jan. 10, 1966,Ser. No. 519,602 Claims. (Cl. 34-5) ABSTRACT 0F THE DISCLOSURE A freezedrying system characterized by direct freeze drying contact betweenliquid feed and a chilled carrier gas at sub-atmospheric pressure. Thecarrier gas recirculates through a cycle in which the carrier gas andthe vapors evolved by the freeze drying are compressed, thenlprogressively chilled to a sub-freezing temperature level. The evolvedvapors condense during the course of Chilling, either being removed as aliquid condensate or deposited in frozen state in the system. Thechilled compressed carrier gas is expanded to further reduce thetemperature level, then is employed to chill the compressed carrier gasby indirect heat exchange therewith and to freeze dry the liquid feed bydirect Contact therewith. Periodically, the feed is halted, and thesystem is regenerated by owing relatively hot gas directly from thecompresser through the ice laden portions of the system to melt frozencondensate.

The present invention relates to a freeze-drying system of highefficiency with low power requirements.

Conventionally freezedrying procedures are affected by subjecting thestarting material to high vacuum conditions and to temperaturessufficiently low to freeze the material. Sublimation reduces the watercontent of the frozen material to the desired freeze-dried level. Theevolved water vapor is compressed and thereafter condensed, usually tosolid ice, in a refrigerated chamber.

Heat transfer to the frozen solid is poor largely because the thermalconductivity of the gas is negligible at the relatively low vacuum whichis maintained in the freeze dry chamber, and because all heat necessaryfor sublimation of the ice must be conducted through the frozen solid.In consequence, such freeze dryers ordinarily operates at a smallfraction even of the equilibrium vapor pressure of ice, the absolutepressure frequently being as low as tenths or hundreths of a millimeterHg.

yMaintenance of freeze drying conditions and removal of the evolvedwater vapor involves high compression ratios (in the thousands).

Characteristically conventional freeze drying features requireconsiderable power, time and labor, making for relatively high costoperation. The principal industrial applications of freeze drying,therefore, are relatively high priced materials, e.g., penicillin.

The principal object of the present invention is to provide an efficientlow cost freeze drying technique.

Further objects and the advantages of the present invention will becomeapparent from the description which follows.

Briey stated the present invention involves freeze drying a pre-chilledliquid feed material by contact with a cold recirculating inert gas. Thegas and evolved water vapor is warmed to about ambient temperature, thencompressed to 12S-3.0 times the absolute pressure of the freeze dryingstep. Thereafter the compressed gas is chilled progressively to asuitably low subfreezing ternperature. During the course of chilling,the evolved water vapor content in the recirculating inert gas iscondensed (principally to ice). The chilled, now cold, gas is thenexpanded essentially adiabatically in an expander engine to theoperating pressure of the freeze drying step. 'The ice expansion servesto recover an appreciable fraction of the power originally required tocompress the gas and to lower the gas temperature still further. Thetemperature differential achieved by low temperature expansion providesthe necessary temperature differential to operate the entire freezedrying system.

The expanded cold gas is warmed to about 10-30 F. by heat exchangeagainst the higher pressure gas being progressively chilled thereby;thereafter the still cold gas directly contacts the material to befreeze dried, removing its water content therefrom as water vapor.Finally the gas and water vapor is again passed in heat exchange withthe freshly compressed gas, thereby being warmed to the about ambienttemperature of compression.

Employment of the above described inert gas cycle improves thermalconductivity of the gas phase, significantly increasing the sublimationrate of ice. The efuent gas and water vapor leaves the freeze drychamber at a ternperature near the freezing point with a water vaporpartial pressure of several millimeters, approaching even thetheoretical of 4.58 millimeter Hg- In addition, a surprisingly lowcompression ratio, e.g., 1.5, is all that is required to operate thefreeze drying system. Moreover, as much as half of the work ofcompression is recoverable in the expander engine. Overall the powerrequirements of the present system are almost negligible compared t0other freeze drying procedures.

Potentially the present procedure is applicable to all concentrationproblems where prolonged exposure t0 temperature levels at which thesolvent (it need not be Water) in liquid phase has deleterious effect onthe concentrated product. In food products, dehydration in the continuedpresence of water (liquid phase) frequently alters flavor or taste; inessence or perfumes so drying affects the aroma; and in vitamins andpharmaceuticals drying at liquid phase temperatures frequently resultsin some product decomposition with loss of potency.

For further understanding of the present invention reference is now madeto the attached drawing wherein is shown, diagrammatically, a flow sheetembodying a mode of the present procedure and which is described belowin terms of a specific example of a preferred ernbodiment of practicethereof. For clarity the exemplary temperature levels have beenillustrated on the drawing.

Recirculating inert gas, suitably air (62 F.) is compressed from 0.5atmospheres absolute to 0.75 atmospheres absolute in compressor 10 (atthe rate of 12,100 lbs. per hour). A major portion of the heat ofcompression is removed (by heat exchange against cooling water) incooler 12, eg., cooled from F. to 80 F. The recirculating compressed airwill be about only 35% saturated (with water vapor) under theseconditions. It is cooled to 48 by exchange against the effluent from thespray dryer in exchanger 14 and then cooled further in the upper section50 of exchanger 18 to 34 F. where it is removed, passed through settlingtank 16 to remove liquid water and then returned to exchanger 18 forfurther cooling in lower section 51 to 20 F. During this stage of thecooling, ice will build up rapidly in heat exchanger 18 since the bulkof the water vapor initially present in the recirculating compressed gasis removed in this section 51 of heat exchanger 18 during the course Ofchilling the gas from about 34 F. to the subfreezing level (-20 F.). Inthe present exemplary instance 10.9 lbs. per hour of water are removedfrom settler 16, and 61.8 lbs. per hour of ice build up in lower section51 of heat exchanger 18.

The chilled gas leaving heat exchanger 18 is expanded under load inexpander engine 20 to approximately 0.5 atmospheres (absolute). T hework recovered in expander engine 20 is employed to supply some of theload of the 3 compressor 10. The snow formed during the essentiallyadiabatic expansion in expander engine 20 is collected in settling tank22 (2.3 lbs. per hour) and the expanded cold 'air (now about 40 F.)passed back into heat exchanger 18 wherein the cold gas is incountercurrent heat exchanger relation to the higher pressure air beingchilled. As shown in the drawing, heat exchanger 18 is constructed sothat the cold gas passes also in countercurrent direct Contact lwith thefreeze dried solids. The freeze dried solids are withdrawn via outlet 19to collection chamber 21. From collection chamber 21 the freeze driedsolids can be removed intermittently without destroying the partialvacuum (1/2 atm.) inside heat exchanger 18.

The cold gas stream passing through heat exchanger 18 is warmed byindirect heat exchange against the higher pressure gas stream and thenby direct contact with the freeze dried solids causes the ice to sublimeinto the gas stream. A small amount of heat is also transferred directlyto the solids to assist in the sublimation of the ice. Ultimately, t-hegas stream leaves heat exchanger 18 and enters spray chamber 24 warmedfrom 40 F. to 27.5 F.

Desirably, the liquid feed solution (eg. 100 lbs. per hour of coffeeextract, 75% water, 25% solids) entering spray chamber 24 via line 26has been pre-chilled, e.g., to about 40 F. The liquid feed spraying intochamber 24 is quickly cooled by evaporation and by direct heat transferfrom the low pressure gas stream passing through chamber 24. The air andevolved water vapor leaves spray chamber 24 via line 25 at about 30 F.The frozen solids pass down through heat exchanger 18 in direct contactwith the colder gas ultimately leaving the system at the rate of 25libs. per ihour via outlet 19 in freeze dried state at about thetemperature of the incoming cold gas (i.e., -40 F.). Y

The gas 12,100 lbs. per hour air, 76.9 lbs. per hour of water vaporleaving spray chamber 24 via gas outlet 25 passes through heat exchanger14 wherein it is war-med to about 62 F. by heat exchange againstcompressed gas, then to compressor to repeat the gas cycle.

As has already been indicated the present system provides for rapidlyfreeze drying a solution with relatively 'high efliciency. Typically,the gas leaving the spray chamber 14, e.g., air, 90% saturated withwater vapor, contains about 0.01 mol fraction of water vapor therein, apartial pressure of several millimeters Hg, a value ywhich Vapproachesthe theoretical 4.58 millimeter Hg.

A distinct advantage of the present system is its relative lack ofsensitivity to minor unbalances. Thus, for example, in small scale unitsWhere appreciable heat may leak into the system the refrigerationobtained by expansion of the chilled gas in expander engine will createa smaller temperature differential (than 20 F.) between the high and thelow pressure gas streams. However, since the minimum temperaturedifferential theoretically required for operation of the present freezedry system is only 6 F., the heat leak will not seriously affectoperation. As a practical matter, the affect of heat leakage is tochange the operational period before regeneration is required to removeice deposited in the high pressure side of heat exchanger 18.

Mention has already been made of that much of the water vapor taken upin the gas cycle will be deposited out as ice on the high pressure sideof heat exchanger 18. As ice builds up there the outlet pressure andtemperature of the gas being chilled are affected until freeze dryoperations must cease, if only because ultimately ice deposition wouldblock gas flow altogether. Practice of the present invention includesIhalting freeze dry operation periodically for regeneration purposes.Regeneration is affected by shutting off the liquid or slurry feed line26, then sending the hot compressed gases via by-pass lines 35 directlyfrom compressor 10 to the lower section of the heat exchanger 18 (bypassing the cooler 12, heat exchanger 14 and the upper section of heatexchanger 18). Within a few minutes, e.g., l0 minutes, the hot gas willmelt all of the ice in exchanger 18 andy the snow collected in settler22 so it can be drawn off via line 36 to settler 22 as liquid water. Thecold gas from settler 22 is Iheated and recycled to the compressor vialine 37, heater 38. The system may be started up by once again sendingthe circulating gas through cooler 12 and heat exchangers 14, 18 andexpander engine 20 and then when temperatures have reached the desiredlevel once again introducing the feed material from inlet line 26.Practice of the current invention on an industrial scale specificallycontemplates having fve or six units in parallel (not shown) so thatcontinuous operation can be obtained with one of the units always onregeneration. Also a vacuum pump (not shown) is needed to create thedesired sub-atmospheric pressure levels in the system.

VThe heat transfer surfaces provided in heat exchangers 14 and 18 shouldbe of the extended surface type with linning virtually essential insection 51 on the high pressure `side of heat exchanger 18. Fins providea sufficiently large surface area for substantial deposition of snow orice formed during cooling of the high pressure stream before the surfacebecomes so insulated by the ice as to reduce heat transfer rates touneconomic limits. If small fin tubes are employed, lower section 51 ofexchanger 18 will become plugged in a relatively short time andautomatically shut down the system by reducing the pressure differentialon the expander engine 20. By way of example, fin tubes about 4 inchesin diameter can operate for about an hour before ice plugging will haltoperation. It should also be recognized that the exclhange surfaceprovided can be in excess of that normally required for noncondensingservice since, as the surface becomes coated with solid it becomesinsulated and the condensation automatically progresses to the followingarea of surface.

Ordinarily the recirculating carrier gas will be air but when thematerial to .be spray dried is susceptible to air oxidation, employmentof a more inert gas may be advisable. Such gases include, for example,nitrogen, the noble gases (helium, neon, argon, krypton, xenon), orcommon gaseous refrigerants such as the well known Freons. Each havetheir special advantages. The noble gases have a lower specific lheatthan the Freons or nitrogen, but in the case of helium this is partiallyoffset by the increased diffusivity therein of water vapor. On the otherhand the higher molecular weights of Freons give more efficientoperation in a centrifugal compressor, requit-ing less stages, and thisfeature is the major advantage of the Freons, assuming of course thatthey are nonreactive with the freeze dried product. Nitrogen is theleast expensive gas, but is intermediate in its other properties betweenhelium and the Freons. The prime requirement for the carrier gas is, ofcourse, that it be inert to the freeze dried product.

In specific application the procedure of the present invention may beemployed in the freeze drying of food products, which can be made toretain their natural flavor thereby, e.g., coffee, milk, lorange juice,other citrus fruits. The present procedure is contemplated also for thedrying of fruits, vegetables and eggs.

The present procedure is not limited to freeze drying of aqueoussolutions only. Other volatile solvent solutions may be dried in thesame manner, even with solvents which do not solidify to ice at thedesired minimum operating temperatures. Where freezing is involved, theheat exchanger 18 is appropriately divided as illustrated to removeliquid condensate (eg. water) from its upper section 51 at just abovethe freezing point. If the solvent does not freeze at all, the heatexchanger 18 need not be divided and periodic regeneration unnecessary.The drying system could then be a continuous operation.

As has already been indicated, a principal advantage of the presentprocedure is its low power requirements.

Operating between atm. and 1/2 atm. expander engine 20 recovers about40% of the power input to the process, and in consequence only about 0.8kwh. is required per pound of water removed from the feed solution.Operation with the low pressure side at atmospheric pressure and thehigh pressure side at 1.5 atmospheres absolute, about doubles the powerrequirement. On the other hand, operating at lower pressures, i.e., 0.25atmosphere at the low pressure side and 0.375 atmosphere on the highpressure side reduces the power requirement to about half the abovevalue. Accordingly, even with equipment available on an o-the-shelfbasis, freeze drying according to practice of this invention costs farless than conventional spray drying. 1f equipment can be designed tooperate at sufficiently reduced pressures operational costs may bereduced virtually to that of boiling water in a single effectevaporator.

It will be obvious to those skilled in the art that various changes maybe made without departing from the spirit of the invention and thereforethe invention is not limited to what is shown in the drawings anddescribed in the specification, but only as speciiied in the appendedclaims.

What is claimed is:

1. A regenerative freeze dry process which comprises:

(l) recirculating a carrier gas through a cooling-warming cycle whereincompressed -gas is progressively chilled in the cooling part of thecycle from about ambient temperature to a sub-freezing temperature inheat exchanger means by heat exchange against expanded gas, thenexpanded to further cool the gas, the expanded gas being progressivelywarmed in the warming part of the cycle by heat exchange against thecompressed gas to about ambient temperature, then compressed forrecycle;

(2) introducing liquid feed directly into the expanded gas, said gasbeing at a ysub-freezing temperature level, the feed being freeze driedby direct contact with the cold expanded gas, the vapor evolved from thefeed thereby being carried along in the expanded gas through the warmingpart of the cycle, compression and the cooling part of the cycle toultimate condensation during the course of said progressive chilling ofthe compressed gas;

(3) periodically halting introduction of the feed and warming thesub-freezing portion of the heat exchanger means to melt condensate icedeposited therein and removing the liquid condensate.

2. The process of claim 1 wherein liquid condensate is removed from thecompressed gas being cooled at a temperature level close to the freezingpoint thereof, just prior to further chilling of the compressed gas tosubfreezing temperatures.

3. The process of claim 1 wherein the warming-cooling heat exchange iseffected in stages, the rst of the stages chilling the compressed gasnot below the freezing point of the liquid in the feed and wherein thefeed is introduced between stages.

4. The process of claim 1 wherein the compression ratio is about 1.5.

5. The process of claim 1 wherein the feed material is pre-chilled closeto the freezing point of the liquid in the feed. l

6. The process of claim 1 wherein the heat of compression of thecompressed gas is removed by an external source of cooling and whereinthe regenerative warming is effected by passing hot compressed gasesdirectly to the normally sub-freezing portion of the heat exchanger.

7. The process of claim 1 wherein the feed is sprayed into the expandedgas.

8. The process of claim 1 wherein water is the liquid in the feed.

9. The process of claim 1 wherein air is the carrier gas.

10. A drying process which comprises:

(1) recirculating a carrier vgas through a coolingwarming cycle whereincompressed gas is progressively chilled in the cooling part of the cyclefrom about ambient temperature to a low temperature by heat exchangeagainst expanded gas, then expanded to further cool the gas, theexpanded gas being progressively warmed in the warming part of the cycleby heat exchange against the compressed gas to about ambienttemperature, then compressed for recycle;

(2) introducing liquid feed directly into the expanded gas, said gasbeing at a low temperature level, the feed being dried by direct contactwith the cold expanded gas, the vapor evolved from the feed therebybeing carried along in the expanded gas through the Warnung part of thecycle, compression and the cooling part of the cycle to ultimatecondensation during the course of said progressive chilling of thecompressed gas; and

(3) removing the vapor condensate.

11. The process of claim 10 wherein the compression ratio is in therange of about 12S-3.00.

12. The process of claim 1 wherein expansion is effected under load.

13. The process of claim 1 wherein the pressure level of both compressedand expanded gases is sub-atmospheric.

14. The process of claim 3 wherein liquid condensate is removed from thecompressed gases in the second o1 the stages of said warming-chillingheat exchange jusl prior to the chilling of the compressed gas from justabove freezing to sub-freezing temperature levels.

15. The process of claim 1 wherein directly after expansion thereof theexpanded gas is partially warmed to a still subfreezing temperaturelevel prior to introduction of the liquid feed thereinto.

References Cited UNITED STATES PATENTS 3,263,335 8/1966 Kan 34-53,266,169 8/1966 Smith 34-5 3,290,788 12/1966 Seelandt 34-5 3,313,0324/1967 Malecki 34-5 WILLIAM I. WYE, Primary Examiner.

